Variable selective system



Jan. 18, 1944. G. w, GILMAN 2,339,633 y VARIABLE SELECTIVE SYSTEM Filed March 23, 1942 "q s h Lncreawc'I/zg I I I I L IL I A. in cram-hg .ZvK/@ganadas amied l I .BY -WzzdOLO/rd.

ATTORNEY Parental rurais y 2,339,633 VARIABLE smcrrvs sys'rm George w. Gilman. snm min, N. J.. mimi-'m Bell Telephone Laboratories, Incorporated,

New York, N. Y.; n corporation of New York Applieautn Marenza, 1942, semi No. vnaar:

This invention relates to electrical transmission systems and more particularly to electrical wave filters. Still more particularly this invention relates to transmission or lter apparatus suitable for transmitting currentsv having any desired band of frequencies and in any portion of the frequency spectrum.

Electrical wave filters usually comprise recurrent networks transmitting with a single structure any desired band width of frequencies in any desired position inthe frequency spectrum. In

order to change the width of the passrband oi the lter or to change the relative position of the pass band inthe frequency spectrum itis4 the usual practice to re-design the circuits of the filter to meet the new requirements. This type ofV prior art structure is therefore unsuitable for use as an arrangement for freely transmittingv with a single structure any desired, band of frequencies changed from one position to. another in 'the frequency spectrum.

It is an object 'of this invention to provide an arrangement which may be quickly adjusted to transmit a band of frequencies of any desired width.

It is another objectof this invention to provide an arrangement which'may be quickly ad- Justed to change the desired band from one position to another in the frequency spectrum.

It is still'another obiectof this invention to provide an arrangement suitable for changing quickly both the width of the band of frequencies transmitted and its relative position in the Vfrequency spectrum.

The invention will be better understood from the more detailed description hereinafter following when read in connection with the accompanying drawing in which Fig. l shows one form of apparatus for practicing this invention, Fig. 2A illustrates graphs which may be used vto explain the operation of the arrangement of Fig. 1,

Fig. 2B illustrates graphs similar to those ofIFig.V

type. The input circuit L1 is. connected to a modulator MDi' to which an oscillator 01 is also of oscillator O1. This upper side band may be press part of'one of the two side bands and all of 6 Claims. (CL 178-44) connected. the modulator MD; being preferably but not necessarily a balancedmodulator of wellknown type which obtains from theosciliator current and the currents ofcircuit Li, .two side bands of the oscillator current, the oscillator current being suppressed. The output of the modulator MDi is supplied to a band-'filter F1 having iix'edV limits, all frequencies beyond the limits of filter-F1 being suppressed. 'I'he output of filter Fx is then supplied to a demodulatorA DM; along with the current of oscillator 01.

The modulator MDi derives two side bands of the current of oscillator 0i, both side bands having the width of the frequencies of the currents flowing over circuitLr. The lter Fi passes only a certain band of frequencies of one of these side bands, all other frequencies outside the finite limits ofv iilter Fx being suppressed. In other words,'fllter'F1 transmits only a.v part of one of the side bands while suppressing Vnot only the other parts of the same side band but also all of v eliminated by Vany simple selective means (not shown) such as an output transformer of suitable characteristics following demodulator DMi,

or a low-passrillter.l e

Theoutput .of demodulator D M1 i's'then'fed to asecond balancedmodulator Ib'fDzLzlOsetherk vwith current from oscillator Oz vwhose frequency is dilferent from that of oscillator O1. The twoj side bands resulting fromjmodulation, each of. Y.

which has a frequencyl width equal tovthat of i'ilter Fi. are then supplied to Ya illter F2 which may be identical to nlter F1. These nlters may be of any suitable type, such as the crystal filters disclosed in Patentk 2,045,991 granted June 30, 1936, to W. P. Mason. The filter F2 acts to Supthe other side band while passing only the remaining' partiof the first-mentioned side bandV whichis within the limits of filter Fr. The cin'-` rents in the part transmitted by filter Fr are fed to a demodulator DMn along with .the current of oscillator O'z and upon demodulation there appears in the output circuit Le leadingto a load circuit LC a reduced band ci' frequencies in its original position in the frequency spectrum shown) such as a suitable output transformer or low-pass filter.

AsV an example, assume that the frequencies of the currents on line L1 extend from zero to 100 kilocycles perv second. If the oscillator O1 is adjusted by condenser C1 to generate current of 134 kilocycles, then the lower side band resulting from modulation in modulator MD1 would extend. from 34 to 134 kilocycles and the upper side band froml34 kilocycles to 234 kilocycles. If filter 1".'1

passes only those currents of frequencies between or overlapping range of-frequencies as represent- -v ed by u-w. Only the overlapping range-of fre-v circuit.

served that these two filters are connectedinv tandem and that their pass bands have a common yband of frequencies two kilocycles wide extending from`7-9 kilocycles. Horizontal lines through 150 and 156 kilocycles, then a portion of the upper'V side band resulting from modulation will be transmitted by filter F1 and all other frequencies of the upper side band and all frequencies ofthe lower side band will be suppressed. The 150-156 kilocycle band will be demodulated in demodulator DM1 to a band extending from 16 to 22 kilocycles. If oscillator Oz is adjusted by condenser- C1 to generate a frequency of 13,6 kilocycles, then the two side bands resulting from modulation in modulator MD: will extend from 114 to 120 kilocycles andv152 to 158 kilocycles. .The lower of these side bands, being obviously outside the limits of 150-156 kilocyclesof filter Fn, will be entirely suppressed. The upper side band is partly within the range of-filter Fa. The. portion within the lter range extends from 152-156 kilocycles and is freely transmitted while the remaining portion of the upper side band is suppressed. 'I'he 152-156 kilocycle band passed by iilter Fn 'is demodulated in .demodulator DM: to supply to line La a band of frequencies extending from 16 to 20 kilocycles. A band 4 kilocycles wide is thus selected and transmitted to line La to the exclusion of all other frequencies. By changing -the values ulator DM1 will then supply a band of frequenpoints w1 and v1 will intersect lines d and e respectively at points s1 and t1 .thereby determining the oscillator frequencies fz and f1 which may be, forexample, 143 and 14'? kilocycles. These are the frequencies of the currents which oscillators O1 and O1 are required to supply to the circuit of Fig. 1 to select the required band. The demodcies r1.s1 which may, for instance, lie in the range of '7 to 13 kilocycles. The band t1u1 of, for instance, 3-9 kilocycles which is the range of the second tandem lter, overlaps the r1-s1 band inthe range 7-9 kilocycles. Only the '1-9 kilocycle band will therefore reach line La.

It will be observed that the width of the band of frequencies transmitted to the `line La is obtained numerically by subtracting the difference between the oscillator frequencies from the difference between the lower cut-ofi' frequency of filter F1 and the upper cut-off frequency of filter F1. For any given networks employing filters F1 and F1 the line La will receive a band of increasing width,v upto the width of the narrowest lter, by

of the frequencies of oscillators O1 and 02,-anyother-band of any desired width .up to the width of the narrowest filter may be selected and transmitted as will be explained hereinafter.

The graphs of Fig. 2A may be employed to determine the band to be selected by the circuit of Fig. 1. The band depends upon the characteristics of filters F1 and Fs and upon the adjustment of the two oscillator frequencies as will be n explained hereinafter.

The two oblique lines d and e are parallel to eachother and the vertical distance l (=r-s) between them represents the'width of the band passed by filter F1 which maybe, for example, 6 kilocycles. If oscillator O1 is set at a frequency f1, then the band of frequencies fed by. demod-l ulator DM1 to modulator MD: is represented by` l La will now transmit only frequencies lying in the range v-unz Y The circuit betweenvmodulator MD1:YY and de-v modulatonDM1 of Fig. 1 rnay be considered to be'- a filter having a pass band, such as r-sof Fig, 2, the width ofthe pass band being determined by the constants of filter F1 and its position in the frequency spectrum being determinedl by oscillator 01. The circuit between modulator MD: and demodulator DMg may likewise be considered l as a-illterhav'ing a pass band, such as t-u of Fig.

2 which is not necessarily equal yto r-s, the width Y of which is determined bythe constants of filter F2 and its position by oscillator O2. It will be obposite slopes of the parallel lines d and e.

reducing the spacing between the oscillator frequencies. and vice versa. It will also be observed that the position in the frequency spectrum of the band of frequencies transmitted to line La can be obtained numerically by subtracting the frequency of oscillator'01 from the lower cut-oil! frequency of filter F1 and by subtracting the frequency of oscillator Oa from the upper cut-on frequency of filter F2.

Fig. 2B is similar to Fig. 2A except for the op- The graphs of Fig. 2Arare employed when the pass bands of the filters F1 and F2 are in the range of the upper side bands resulting from modulation. The graphs of Fig. 2B are tobe used when the pass bands of the filters F1 and F2 are in th range of the lower side bands.

It will be observed that the frequency fs which is midway between the oscillator frequencies Vf1--f2 shown in Fig. 2A, corresponds, as is shown in the dot-and-dash lines of Fig. 2A, to the midfrequency fm of the band of frequencies which the system is capable of transmitting betweenlines L1 and Ls. It will be noted that as the frequencies f1 and f1.l are varied without altering the difference between them, the circuitof Fig. 1 will supply to line L2 a band o! frequencies of Vconstant widthv-ua but of varying position in the frequency spectrum. Thus, different portions of the incoming frequencies in line L1 may be selected and transmitted to line L2 and there explored or detected.

It will also be noted from Fig. 2A that if the spacing between the oscillator frequencies f1--fz is increased or decreased, the band of frequencies v--w passed to line Lz will be narrowed or widened respectively by corresponding amounts. The widening of the band, however, is limited by the width ofthe narrowest pass band of filters F1 and F2 which, of course, may have any desired limits. By so varying the width of the selected I" bandand its location in the frequency spectrum,

aasacss of the invention and the scope of the appended theoperator may explore any given range of frequencies no matter how wide or narrow.

Fig. 3 shows the mechanical apparatus which may be used to interconnect the condensers Ci and Cz of Fig. l to provide a convenient control for varying both the position of the pass band in the frequency spectrum and its width. The control .may be exercised by operating two dials,`

designated A and B, for varying the condensers .Ci and (la. 'It is assumed that the plates ofthe .variable condensers Ci and Cz are so cut that for each degree of rotation of the rotors of these -condensers, the oscillator frequencies f1 and fz will be changed .by a flxedand predetermined number of cycles.

v In Fig. 3, the condenser ci (of oscillator o1) is adjusted directly by turning the dial A. Any rotation of dial A also adjusts the capacity of condenser Cz (of oscillator Oz) in the same direction and through the same angle. The simultaneous adjustment of condenser C2 is accomplished by the gear mechanism of the differential gear box D which is coupled to the gears designated E. 'I'he reversing gears E are linked to condenser C: as shown.` Thus the gear mechanism just referred to serves to impart to condensers Ci and C: simultaneous adjustments in capacity in the same di.-

rection, the amount of which is determined by Vthe angular displacement of dial A.

The housing S of the differential gear box D is rotatable about the main shaft of condenser Ci and dial lA as shown in Fig. 3. The dial B is shown coupled by gears F and G tothe housing S. By rotating'dial B the housing S is rotated in the same direction thereby causing rotation of the rotors of condenser C2 relative to the rotors of condenser Ci. Thus dial B may be manipulated to vary the width of the band of frequencies (v-wi selected by the circuit of Fig. l1.

After dial B has been operated to enect the selection of the width of the band of frequencies which the system shown in Fig. 1 will be capable of transmitting between lines lLi and In, the selected band width can be readily moved up and down the frequency spectrum by merely operating -dial A. As the dials A and B can be operated independently of each other, a band of any desired width up to the width of the narrowest filter can thus be selected and moved up and down the frequency spectrum as desired. The filters F1 and F2 need not have substantially the same fixed limits. Filter Fa should have an upper cut-oil frequency which is a denite band width above the lower cut-off frequency of filter F1. The filters F1 and F2 should be declaims.

`What is claimed is: l

Vl. A variable selective system for selecting and transmitting only a desired band of frequencies to the exclusion of all other frequencies, said system comprising twofllter networks connected in tandem, eachnetwork including a band illtertogether with an oscillator and means for changing' the position in the frequency spectrum of received currents impressed on the associated filter, said networks being adapted to pass bands of fre'` quencies which overlap in the range ofthe desired band of frequencies, the frequencies of said oscillators being different and adjustable, a diiferf ential gear box for controlling the frequencies of said oscillators for varying both theA width and the location in the frequency spectrum of the desi redband, and two operating means for operat Y ing said gear box, one of said operating means being adapted to change the difference between the oscillator frequencies for changing the width of the band of Vfrequencies transmitted through said networks by an amount corresponding to the change in the difference between said oscillator frequencies, and thev other of said operating means being adapted to change the .frequencies of both oscillators equally and simultaneously in the same direction for changing the relative position in the frequency spectrum of the band of frequencies transmitted through said networks by an amount corresponding to the change inthe oscillator frequencies.

2. A variable selective-system comprising means for obtaining two bands of frequencies which partially overlap in the frequency spectrum, means signed to substantially suppress the unwanted products of modulation, such as undesired side bands.

The circuits and mechanism of this invention may. be employed, for example, to analyze frequency components of portions of a complex signal. It may also be used to distinguish a set of vibrations or oscillations from any other vibrations or oscillations.

esv

The frequencies referred to hereinabove have fortransmitting only the overlapping portion of said two bands of frequencies to the exclusion of all other frequencies, first control means for selecting and varying the width of the overlapping portion of said two 4bands of frequencies, and second control means for independently and separately selecting and varying the position in the frequency spectrum of the overlapping portion of said frequencies whose width was selected by saidfirst control means.

3. A selective system for selecting and trans- .mitting any desired band of frequencies from a complex source to the exclusion of all other fre quencies, comprising means for changing all of the-frequencies of said source to a different position inthe frequency spectrum, a first b and filter having a pass band ofsmaller range than the range of said changed `frequencies for transmitting only those changed frequencies which come within the limits thereof, means for again' changing the position in the frequency spectrum o f said transmitted frequencies, and a second band filter for transmitting onlyV those frequencies obtained from rsaid second means which come within the limits of said second filter, said` band filters being adapted to transmit bands oi frequencies which'overlap in the desired band of frequencies. f

4. Selective apparatus comprising two oscillators of variable frequency, a paire! iilter networks connected .in tandem and each having a modulator connected to its'input side and a demodulator connected to its output side, and circuit means for connecting one of said oscillators to one of said modulators and its associated demodulator and for connecting the other of said oscillators to the other of said modulators and its associated demodulator for selecting a band of frequencies quency spectrum is determined bythe frequencies of the oscillators; Y.

5. A variable band selective system for selecting and transmitting only a desired band of frequencies to the exclusion of all other frequencies, saidsystem including a source of currents, two selective networks connected in tandem, each network comprising a modulator and a iilter and a demodulator all connected in tandem, two variable Y oscillators, first coupling means for coupling one of said oscillators to both lthe modulator and the demodulator in one of said networks, second coupling means for coupling the other of said oscillators to both the modulator and the demodulator in the other of said networks, means for applying currents from said source to one of said networks, and control means for varying the frequencies of said oscillators. 6. A variable band selective system for selectwhose width and position in the frev ing and transmitting only a desired band of frequencies tothe exclusion of all other frequencies, said system including a source of currents, two selective networks connected in tandem, each network comprising a modulator and a illter and a demodulator all connected in tandem, two variable oscillators, first coupling means for coupling one of said oscillators to both the modulator and the demodulator inV one of said networks. said coupling means for coupling the vother of said oscillators to both the modulator and the demodulator in the other of said networks, means for applying eurrentsfrom said source-to one of said networks, tlrst control means for simultaneously varying the frequency of each of said oscillators, and second control meansfor separately varying the frequency of one of said oscillatorsL GEORGE W. GILMAN. 

